Radar system using random RF noise

ABSTRACT

A radar system that utilizes predetermined, pseudorandom, or random waveforms that may be substantially matched to the impulse response of the radar and any surrounding clutter such that the signal-to-clutter ratio may be optimized and/or such that specific targets may be identified and/or classified.

The present invention was made with United States Government supportunder contract no. SUBK4400039015. The United States Government may havecertain rights to this invention under 35 U.S.C. § 200 et seq.

BACKGROUND OF THE INVENTION

The present invention relates generally to radars. More particularly,the present invention relates to a noise radar. For instance, anexemplary embodiment of the present invention may use transmitted noisein the radio frequency (RF) range. As used herein, the term “noise”shall be understood random noise as well as pseudorandom noise.

Radars are used for a variety of purposes. For example, radars are usedto detect the speed at which a target (e.g., a vehicle, a ball, aperson, etc.) is traveling. Radars, such as in security systems, arealso used to detect other types of movement. In addition, radars arealso used to determine the presence of a target.

Despite the many benefits, conventional radars have a number ofsignificant drawbacks. For instance, obstructions such as buildings,foliage, and other types of structures or vegetation may impede theperformance of a radar by blocking or deflecting the signals transmittedby the radar. In addition, the signals used by other types of electronicdevices may interfere with the performance of radar. As might beexpected, the signals transmitted by the radar can also interfere withthe functioning of other electronic devices. Furthermore, conventionalradars may not be adapted to identify or classify specific types oftargets.

In light of the drawbacks of conventional radars, a need exists for animproved radar that optimizes the signal to clutter ratio. A need alsoexists for an improved radar that is adapted to identify and/or classifyspecific targets. Yet another need exists for an improved radar that isless susceptible to interference from other electronic devices. Afurther need also exists for an improved radar that is less likely toimpede the performance of other electronic devices.

SUMMARY OF THE INVENTION

The present invention is a radar that may use transmitted radiofrequency noise. However, it should be recognized that some embodimentsof the present invention may use transmitted signals outside of theradio frequency range. An exemplary embodiment of the present inventionmay utilize predetermined, pseudorandom, or random waveforms that may besubstantially matched to the impulse response of the radar and anysurrounding clutter such that the signal-to-clutter ratio may beoptimized and/or such that specific targets may be identified and/orclassified.

One exemplary embodiment of the present invention is a radar system.This embodiment of the radar system includes a memory device that isadapted to store a first radio frequency waveform. Also, a second memorydevice is adapted to store a second radio frequency waveform. A timingdevice may be in communication with the first memory device and thesecond memory device. The timing device is adapted to delay an output ofthe second radio frequency waveform relative to the first radiofrequency waveform. A mixer is then adapted to mix the delayed output ofthe second radio frequency waveform with a signal that is derived fromthe first radio frequency waveform interacting with a target.

Examples of the first memory device and the second memory device includesequential memory devices. A first-in/first out circuit (FIFO) is oneexemplary embodiment of a sequential memory device. However, it shouldbe recognized that the first memory device and the second memory devicemay not be identical components.

The radio frequency waveforms may be wide band radio frequencywaveforms. In addition, the waveforms may be random, pseudorandom, orpredetermined. In one exemplary embodiment of the radar system, theradio frequency waveforms may each have a bandwidth of at least about100 MHz. For example, the bandwidth may be between about 100 MHz andabout 10 GHz (e.g., about 500 MHz or about 800 MHz). However, it shouldbe recognized that the radio frequency waveforms may each have abandwidth greater than about 10 GHz.

The radar system may also include a low pass filter adapted to filter anoutput of the mixer. Additionally, the radar system may comprise atleast one antenna that is adapted to transmit the first radio frequencywaveform (or the output of a filter for the first radio frequencywaveform) and to receive the signal that is derived from the transmittedsignal interacting with the target. An exemplary embodiment of the radarsystem may also include a computer in communication with the firstmemory device and the second memory device, wherein the computer isadapted to determine the first radio frequency waveform and the secondradio frequency waveform. Also, a first filter may be in communicationwith the first memory device such that it is adapted to filter the firstradio frequency waveform. Similarly, a second filter may be incommunication with the second memory device such that it is adapted tofilter the second radio frequency waveform. For example, the firstmemory device and the second memory device may each be a 1-bit wideFIFO. An exemplary embodiment of the radar system may also include arespective D/A converter in communication with each memory device. OneD/A converter may be adapted to convert the first radio frequencywaveform, and the other D/A converter may be adapted to convert thesecond radio frequency waveform. Furthermore, an oscillator may beincluded that is adapted to produce a third radio frequency waveform. Asecond mixer may be in communication with one D/A converter and theoscillator. The second mixer may be adapted to mix an output of the D/Aconverter with the third radio frequency waveform. In addition, a thirdmixer may be included that is in communication with the first mixer andthe oscillator, wherein the third mixer is adapted to mix the thirdradio frequency waveform with a signal that is derived from an output ofthe second mixer interacting with the target. In such an embodiment, thefirst mixer may be adapted to mix an output of the second D/A converterwith an output of the third mixer.

Another exemplary embodiment of the radar system may include a 1-bitwide FIFO memory device that is adapted to store a first radio frequencywaveform. A second 1-bit wide FIFO memory device may also be includedthat is adapted to store a second radio frequency waveform. A timingdevice may be in communication with the first memory device and thesecond memory device. The timing device is adapted to delay an output ofthe second radio frequency waveform relative to the first radiofrequency waveform. A first filter may be in communication with thefirst memory device such that it is adapted to filter the first radiofrequency waveform. Similarly, a second filter may be in communicationwith the second memory device such that it is adapted to filter thesecond radio frequency waveform. A mixer may be included that is adaptedto mix an output of the second filter with a signal that is derived froman output of the first filter interacting with a target. Additionally,this embodiment of the radar system may include any of the optional orpreferred features of the other embodiments of the present invention.

In yet another embodiment of the radar system, a first memory devicestores a first radio frequency waveform, and a second memory devicestores a second radio frequency waveform. A timing device may be incommunication with the first memory device and the second memory device.The timing device may be adapted to delay an output of the second radiofrequency waveform relative to the first radio frequency waveform. Afirst D/A converter may be in communication with the first memorydevice, wherein the first D/A converter is adapted to convert the firstradio frequency waveform. Similarly, a second D/A converter may be incommunication with the second memory device such that it is adapted toconvert the second radio frequency waveform. An oscillator may beprovided that is adapted to produce a third radio frequency waveform. Afirst mixer may be in communication with the first D/A converter and theoscillator. The first mixer may be adapted to mix an output of the firstD/A converter with the third radio frequency waveform. Additionally, asecond mixer may be in communication with the oscillator. The secondmixer may be adapted to mix the third radio frequency waveform with asignal that is derived from an output of the first mixer interactingwith a target. Furthermore, a third mixer may be in communication withthe second D/A converter and the second mixer, wherein the third mixeris adapted to mix an output of the second D/A converter with an outputof the second mixer. A low pass filter may be included that is adaptedto filter an output of the third mixer. Also, at least one antenna maybe adapted to transmit an output of the first mixer and to receive thesignal that is derived from the output of the first mixer interactingwith the target. This embodiment of the radar system may also includeany of the optional or preferred features of the other embodiments ofthe present invention.

It should be recognized that each memory device of the present inventionmay be used to store a plurality of waveforms. The waveforms may beoutput in a predetermined order to facilitate the identification of avariety of different targets. For one example, each noise waveform maybe predetermined to have certain characteristics (e.g., frequency,amplitude, etc.) which facilitate the identification of a particulartarget of interest. One such exemplary radar system comprises a firstmemory device adapted to store and output at least one waveform and asecond memory device adapted to store and output at least one waveform.In particular, the first memory device and/or the second memory devicemay be adapted to store and output a plurality of different waveforms.In one exemplary embodiment of this radar system, the first memorydevice may be adapted to store a plurality of different waveforms andoutput them in a predetermined order. In another exemplary embodiment,the second memory device may be adapted to store a plurality ofdifferent waveforms and output them in a predetermined order. A timingdevice is in communication with the first memory device and the secondmemory device. The timing device is adapted to delay an output of thesecond memory device relative to the output of the first memory device.Thereafter, a mixer is adapted to mix the output of the second memorydevice with a signal that is derived from the output of the first memorydevice interacting with a target. It should be understood that thisembodiment of the radar system may include any of the optional orpreferred features of the other embodiments of the present invention.

A radar system of the present invention may also include more than twomemory devices. This is an example of another type of system of thepresent invention which may facilitate the identification of a varietyof targets. In particular, the output of each memory device may be apredetermined waveform having certain characteristics that facilitatethe identification of particular targets of interest. For example, theremay be a plurality of memory devices that are adapted to outputwaveforms to interact with a target, and/or there may be a plurality ofmemory devices that output waveforms to be mixed with a signal reflectedfrom a target.

In one exemplary embodiment of this radar system, a first memory deviceis adapted to store and output at least one waveform. There are aplurality of additional memory devices, each adapted to store and outputat least one waveform. A timing device is in communication with thefirst memory device and the additional memory devices. The timing deviceis adapted to delay each output of the additional memory devicesrelative to an output of the first memory device. Multiple mixers arealso included. Each of the mixers is adapted to mix the output of arespective one of the additional memory devices with a signal that isderived from the output of the first memory device interacting with atarget. This embodiment of the radar system may include any of theoptional or preferred features of the other embodiments of the presentinvention.

In another example of the radar system, a first memory device is adaptedto store and output at least one waveform. The radar system alsoincludes a plurality of additional memory devices, each adapted to storeand output at least one waveform for interacting with a target. Theadditional memory devices may be adapted to produce outputsimultaneously or in a predetermined order. A timing device is incommunication with the first memory device and the additional memorydevices. The timing device is adapted to delay an output of the firstmemory device relative to an output of one of the additional memorydevices. A mixer is adapted to mix the output of the first memory devicewith a signal that is derived from the output of one of the memorydevices interacting with a target. In addition, this embodiment of thepresent invention may include any of the optional or preferred featuresof the other embodiments of the present invention.

In addition to the novel features and advantages mentioned above, otherfeatures and advantages of the present invention will be readilyapparent from the following descriptions of the drawings and exemplaryembodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of an exemplary embodiment of a matched-waveformnoise radar of the present invention.

FIG. 2 is a diagram of an exemplary embodiment of a matched-waveformnoise radar of the present invention which uses up/down RF conversion.

FIG. 3 is a diagram of an exemplary embodiment of a matched-waveformnoise radar of the present invention which uses RF filters.

FIG. 4 is a diagram of another exemplary embodiment of the radar systemof the present invention.

FIG. 5 is a diagram of an exemplary embodiment of a correlation-typereceiver that may be utilized in each channel of the radar of thepresent invention.

FIG. 6 is a schematic of an exemplary receiver module of the presentinvention.

FIG. 7 is a block diagram of an exemplary embodiment of a PRN generator.

FIG. 8 is a schematic diagram of an exemplary embodiment of a Galoisword generator.

FIG. 9 is a block diagram of an exemplary embodiment of a diplexermodule.

FIG. 10 is a block diagram of an exemplary embodiment of an upconverter.

FIG. 11 is a circuit diagram of an exemplary PC board layout.

FIG. 12 is an outline of an exemplary Vivaldi TSA antenna horn section.

FIG. 13 is a block diagram of an exemplary embodiment of a dataacquisition process.

FIG. 14 is a flow diagram of an exemplary embodiment of a set and recordsubroutine for data acquisition hardware control.

FIG. 15 is a timing diagram for an exemplary embodiment of a hardwarecontrol subroutine.

FIG. 16 is a diagram of another exemplary embodiment of amatched-waveform noise radar of the present invention which produces aplurality of outputs.

FIG. 17 is a graph of exemplary outputs which may be produced by theradar system of FIG. 16.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENT(S)

The present invention is directed to a radar that may use transmittedradio frequency noise. An exemplary embodiment of a matched-waveformnoise radar of the present invention is shown in FIG. 1. In thisexample, a wide band radio frequency waveform may be stored in a memorydevice such as a sequential memory device. An exemplary embodiment ofthe waveform may be carrier-free and designed for a specific target(including target distance) and clutter conditions. In addition, anexample of the waveform may operate over a broadband of frequencies andhave a low voltage, thus enabling the use of microchips and othersimilar or related electronics. The waveform is radiated by at least onetransmitter antenna. The waveform may propagate to a target in what maybe a complex, perhaps dispersive, environment. The waveform may interactwith the target of interest as well as other objects (i.e., clutter),thereby changing the waveform. The new, reflected waveform may be theconvolution of the antenna and propagation environment transfer functionas well as the radar target and radar clutter scattering transferfunction (i.e., the target and clutter impulse response). Meanwhile,another input waveform may be stored in a second memory device. Thissecond input waveform may be substantially similar or dissimilar to thetransmitted waveform. In an exemplary embodiment of the presentinvention, the second input waveform may also be designed for a specifictarget (including target distance) and clutter conditions. The secondwaveform may be clocked delayed and sent to a RF mixer. The reflectedsignals from the target and clutter are also transmitted to the RF mixerby at least one receiving antenna (which may be in the same or differentarchitecture as the transmitting antenna). For optimum correlation ofthe signals in an exemplary embodiment of the present invention, thesecond waveform may substantially match the desired portion of thereflected waveform from the target and clutter. The output of the mixermay then be filtered (e.g., low-pass filtered) and transmitted as aslowly varying voltage waveform to a processing device. For example, asystem computer may be used to analyze the voltage waveform to identifyand/or classify the target.

It should be recognized that the resultant voltage waveform may be thereal time radar cross correlation coefficient. The combination of the RFmixer and the filter may perform real time analog correlation. As aresult, the delay function for the reference signal (i.e., the secondinput waveform) may extend this function to real time analog crosscorrelation. The second input waveform may be synthesized such that thecross correlation achieves a number of desired functions. For example,the second input waveform may be selected to produce a maximum crosscorrelation with the signal scattered from a specific type of target(i.e., target of interest). As a result, the radar of the presentinvention may be optimized for detecting a specific target of interest.Examples of targets include, but are not limited to, those that may bedetected by known radar systems. For instance, the radar system may beused to detect people, moving objects, stationary objects, metalobjects, pipes, bars, trees, curbs, fire hydrants, telephone poles,fences, cinder blocks, pot holes, weapons, vehicles, tumors, and anendless number of other types of objects. In addition, the radar of thepresent invention may be used for night vision. Furthermore, the secondinput waveform may be designed to maximize the correlation response froma target of interest while also minimizing the response from the clutter(such as buildings, grass, weeds, trees, etc.). In fact, many otherdesigns are possible. Special pairs of the input waveforms may bedesigned to perform desired functions including, but not limited to,target identification and classification, target-to-clutteroptimization, and to compensate for the dispersive effect of complexpropagation environments (such as through walls of buildings, throughfoliage, or under ground, etc.).

The particular bandwidth of an input waveform may be selected tooptimize the signal to clutter ratio and/or to facilitate theidentification and/or classification of a specific target. The centerfrequency is in the desired radar band for the particular purpose of theradar. The particular frequency band may be selected based on a varietyof factors including, but not limited to, the type of targets, the typeof clutter, component (including antenna) specifications, the desireddegree of resolution, and other factors which relate to the performanceof the system. As noted above, the radar-transmitted signal may be awaveform with a frequency spectrum that extends over a wide band offrequencies. The frequency spectrum may range from about 0 Hz tofrequencies in excess of 10 GHz. For example, an input waveform may havebandwidth of at least about 100 MHz. For instance, an input waveform mayhave a bandwidth in the range from about 100 MHz to about 10 GHz (e.g.,about 800 MHz). Some exemplary embodiments of an input waveform may havea bandwidth in the range from about 100 MHz to about 500 MHz or fromabout 300 MHz to about 350 MHz. Nevertheless, in certain embodiments ofthe present invention, the bandwidth of an input waveform may be lessthan about 100 MHz or greater than about 800 MHz or even greater thanabout 10 GHz.

Another exemplary embodiment of the present invention is shown in FIG.2. Optional amplifiers are omitted for clarity. This embodiment of thepresent invention utilizes up/down RF conversion. Two high-speed FIFOmemory devices are used to store the input waveforms. The FIFO memorydevices may be digital chips that hold an array of data that may besequentially clocked out of memory. The memory is clocked out in thesame order in which it is loaded (rather than randomly accessed). Forexample, an example of a FIFO memory device may have more than about32,000 nine-bit words available with a speed greater than about 100 MHz(e.g., greater than about 300 MHz). A D/A converter may be incommunication with each memory device. The speeds of any associated D/Aconverters may run at about the same speeds as the memory devices.However, a computer may load the data into the memory at a relativelylow speed. After the specially designed input waveforms are loaded intomemory, the clock may be enabled, and the memory may run, for example,at 100 MHz or more. Differential delays may be implemented using delaytriggers with the clock or by using a built-in offset address setting inthe memory device. An oscillator may be provided that is adapted toproduce another input radio frequency waveform. A first mixer 20 may bein communication with a D/A converter and the oscillator. The firstmixer 20 may be adapted to mix an output of the D/A converter with theinput waveform from the oscillator. Additionally, another mixer 22 maybe in communication with the oscillator. The second mixer 22 may beadapted to mix the input waveform from the oscillator with a signal thatis derived from an output of mixer 20 interacting with a target.Furthermore, a third mixer 24 may be in communication with the other D/Aconverter and the mixer 22, wherein the third mixer 24 is adapted to mixan output of the second D/A converter with an output of the mixer 22. Alow pass filter may be included that is adapted to filter an output ofthe mixer 24. Also, at least one antenna may be adapted to transmit anoutput of the mixer 20 and to receive the signal that is derived fromthe output of the mixer 20 interacting with the target. The radar may berelatively small and low cost. In one exemplary embodiment of this radarsystem, the radar was able to track a person carrying a radar reflectorto a distance of more than 170 feet using less than {fraction (1/10)}watt of power.

Another embodiment of the present invention may implement directfiltering of the outputs of the memory devices. FIG. 3 shows an exampleof such an embodiment. This embodiment of the present inventionrecognizes that although the clock speed of the memory devices may begreater than about 100 MHz, the RF signal bandwidth may have componentsthat extend to at least more than 10 times that because of the rise timeof the pulses. Accordingly, this example uses only one-bit serial data(i.e., the output may be a single set of voltage levels representing thedesired waveforms). The RF filters may then select the bandwidth overwhich the system is to operate and extract the desired components of thememory output. The output signals may need to be significantlyamplified. However, the amplifiers are not shown for clarity. In such anembodiment, the new, reflected waveforms from the target may not belinearly related to the second input waveform. Accordingly,consideration must be given to this fact when designing the waveforms tooptimize signal-to-clutter ratio or to classify a specific target.

As mentioned above, a radar system may be used to detect multipletargets. For example, FIGS. 2 and 3 illustrate radar systems in whicheach of the memory devices may be used to store a plurality of speciallydesigned waveforms. A timing device may then be used to output thewaveforms in a predetermined order. It should also be recognized thatany of the aforementioned embodiments of the radar system of the presentinvention may incorporate additional memory devices. FIG. 16 shows anexample of a radar system that includes a plurality of memory devicesthat are adapted to store and output waveform #2, waveform #3, waveform#4, and waveform #5, respectively. These waveforms may be outputsimultaneously. Alternatively, these waveforms may be clocked out in apredetermined order. These waveforms may be delayed relative to waveform#1. After waveform #1 reflects from a target, the reflected signal maybe mixed with waveform #2, waveform #3, waveform #4, and waveform #5,respectively. In one exemplary embodiment, each pair of signals may bedesigned to identify and/or classify a specific target whencross-correlated. The resulting outputs (i.e., output #1, output #2,output #3, and output #4) may be then be monitored to determine if theradar system detected any of the specific targets. For example, FIG. 17shows a graph of an exemplary output #1, output #2, output #3, andoutput #4 versus time/distance. In this example, output #3 detected aspecific target at a particular time/distance as indicated by theimpulse response peak.

It should be recognized that a radar system of the present invention mayalso include a plurality of waveforms for interacting with a target. Inthe exemplary embodiment of FIG. 16, the radar system includes anothermemory device for producing waveform #6. Waveforms #1 and #6 may beoutput simultaneously or in a predetermined order. Each of thesewaveforms may be specifically designed to facilitate the detection of aparticular target of interest. After interacting with a target ofinterest, the resulting signal may be cross-correlated with one or moreother waveforms to detect the target of interest.

EXAMPLES

An exemplary embodiment of a stealthy noise radar for vehicle obstacleavoidance was built. This example of the noise radar achieved stealth byusing a noise-like (pseudorandom) waveform with more than 700 MHz ofbandwidth. The radar used parallel analog correlation cross correlationfor receiving. This provided protection from jamming (accidental due tolocal communications transmitters or deliberate) as well as mitigationof interference caused by the radar. Several radars of the same designcan be operated in the vicinity of each other without interfering eachother.

The radar was mounted on a stationary support and tested in a foliageenvironment. It showed that it can detect, range, and/or classifyobstacles such as humans, cinder blocks, metal cylinders, various sizetrees, tree stumps, rocks, and holes. In particular, this embodimentcould detect humans to 250 feet and small trees to 350 feet.

The radar, for example, may use a single steered-beam antenna or amulti-beam antenna. A multi-beam antenna may reduce cost and antennacomplexity with no reduction in performance. With this antennaconfiguration, the system can operate as a multi-element array allowing2-dimensional imaging of the radar scattering field in the foreground ofthe vehicle.

Preliminary Data Using a Frequency Domain Radar

A swept-frequency or step-frequency radar (i.e., a frequency domainradar) was used to study propagation and imaging. Although a frequencydomain radar is a substantially different system than a noise radar ofthe present invention, the basic bandwidth and propagationcharacteristics may be about the same for a noise radar of the presentinvention. As a result, this preliminary testing supported thecapability of a noise radar of the present invention.

Two imaging methods were used during the preliminary testing torepresent the collected array data as a 2-D array of radar scatterers ina crossrange vs. downrange image. These methods are, the “direct”imaging method or the 2-Dimensional Inverse Fast Fourier-Transform(2D-IFFT), and the backprojection algorithm. These two methodscompliment each other in speed and resolution, hence creating anapplication-specific need for each. However, a 2D-FFT algorithm requiresmeasurement in the frequency domain. On the other hand, a noise radarmay operate in the time domain.

With the use of both algorithms, the method of collecting data and thepreliminary math may be similar. Data may be collected in a 2-D array asa function of frequency and crossrange antenna movement. An inverseFourier-transform may be performed on each column of frequency data,creating a range-profile. It should be noted that data could also becollected in the time-domain using a noise radar of the presentinvention, eliminating the processing time needed to perform thisinitial transformation.

In the 2D-FFT method, a row-wise Fourier Transform was performed on therange-profile, which condensed the streaks seen in the display image ofthe data into localized scattering centers in real cross-range position.Fast algorithms for these procedures are common and allow near real-timeimaging. The main drawback of this method may be the assumption that allscatterers in the image are in the far-zone of the antenna. When thisrequirement is not met, the scattering centers of near-field targets mayappear smeared over the crossrange, while the targets in the far-zoneare suitably focused. For this reason, the backprojection algorithm maybe used primarily to image near-field objects in detail.

Backprojection imaging may be used with the noise radar of the presentinvention. Backprojection is an imaging method that may operate solelyin the time-domain. The time-domain response from one stationary antennarepresents the impulse response from targets in the downrange as afunction of the total delay (antenna-to-target-to-antenna). With datafrom one antenna, it may only be possible to calculate distance to thetarget using $d = {c \cdot \frac{T}{2}}$with c as the speed of light. The target's placement in the crossrangemay be determined through finding the point in the foreground where thescattering responses from two or more antennas intersect. The resolutionof this type of system may depend on the number and spacing of theantennas.

Generally, the procedure may consist of circularly interpolating eachantenna's data, then adding all of the antenna responses. Because ofthis channel-by-channel processing, the backprojection method mayrequire more CPU time than the 2D-FFT method for a frequency domainradar. However, since this algorithm may be based on propagation timeonly, no plane-wave assumptions may be required and targets in thenear-field of an antenna can be imaged accurately.

Imaging from a moving vehicle may be time-intensive and targets ofinterest may be approaching from well outside of the antenna'snear-zone. For these reasons, the 2D-FFT may be a preferred imaging modefor some applications of a frequency domain radar, with thebackprojection method being used when appropriate to its timerestrictions.

Testing with a frequency domain radar was carried out using one antennaon a moving probe to simulate an array of many antennas. A computersystem precisely set each antenna position and recorded frequency datafrom a network analyzer. The network analyzer measured transmissioncharacteristics- with a 2-18 GHz AEL standardized horn antenna attachedto its transmit and receive ports through SMA cables. This analyzer wasinterfaced to the computer system with a GPIB connection. The computersystem was also interfaced to a standard stepper motor controllerthrough a parallel input-output connection. Finally, the stepper motorcontroller sent position feedback information to the motor attached tothe moving probe.

In this example, the probe was set horizontally at a height of fourfeet, and the receive antenna was attached to the moving carriage. Thetransmit antenna was mounted on a tripod centered with the probe, 1 footlower, and 3 inches forward. The computer, network analyzer, and motorcontroller were grouped to the side of the probe 20 feet away.

Tests recorded data over a frequency range of 2-6 GHz with 801discretely sampled frequency points separated by 5 MHz. The receiveantenna was moved to 84 independent probe positions spanning 7 feet andeach separated by 1 inch. Via the Spatial Nyquist Theorem, the physicalspacing may be at most one-half of a wavelength to eliminate the chanceof spatial aliasing. Targets included a live tree, a 2′ tall by 6″diameter metal cylinder, a horizontally placed cinder block, and a 1′deep by 1′ wide pothole. Before testing in clutter, each of thesetargets was scanned in an open field of short grass to determinecharacteristics associated with each.

Using the imaging techniques described earlier, the characteristics ofthe targets were examined. In the absence of intervening foliage, themetal cylinder, cinder block, and tree were clearly detectable, withpower return 4.5 times greater than the background for the tree andcylinder and 11 times greater for the cinder block.

In the absence of foliage, images using both the 2D FFT and thebackprojection algorithm identified some differences in thecharacteristics of targets. Three targets are examined in detail: the 2′metal cylinder, the cinder block, and the tree. These targets differ inshape and composition and show several distinct traits. The cinder blockhad a flat vertical face, which is perpendicular to the incident wavedirection and caused a greater reflection than the curved surface of thecylinder. This was apparent by viewing 2D FFT images of the two targetswith a linear display and the same intensity scales. The return from thecinder block was roughly 2.5 times as great as that from the cylinder.There are also differences in the patterns of the two targets. Thecinder block has its maximum return value localized in a single locationwith lesser returns surrounding this high intensity center. The aluminumcylinder exhibits a more distributed maximum return, which shows greatercurvature, and in this instance, three separate maximum locations.

The tree had approximately the same RCS magnitude as the aluminumcylinder, power levels 4.5 times greater than the background level,while being 46 feet away, more than twice the distance of the cylinderor cinder block. At the same range as the cylinder or cinder block, thetree would show a larger return than either target as a result of itslarger size.

The properties of the cylinder and cinder block described above are alsoapparent in the images created with the backprojection algorithm. Theintensity offset of the cinder block from the background continues toshow a greater value and more concentrated distribution than that of thealuminum cylinder.

Taking the images produced by both algorithms as a whole, thepreliminary results indicated that features of the targets are clearlyidentifiable and may be built upon to distinguish betweeninconsequential targets and those of particular interest. Although thebasic bandwidth and propagation characteristics were tested using afrequency domain radar, these test results are significant because thebandwidth and propagation characteristics may be about the same for anoise radar of the present invention.

The effect of foliage did not in this case degrade the image of atarget; rather it introduced many extraneous scattering centers. Thesignal processing of the present invention enabled us to identify thetargets using their characteristics and separate them from unimportantscatterers. Examining these images produced with the 2D FFT, thecylinder was clearly visible with a power 4 times greater than thebackground at a distance of 10 ft and 1.6 times greater at a distance of20 ft. The tests showed that the present invention may identify targetswhose line of sight may be obstructed by foliage. Background subtractionmay be used to assist in the identification of the target, particularlyif the target is in close proximity to the foliage scattering centers.

Noise Radar Hardware Design

A block diagram for this exemplary embodiment of the noise radar systemis shown in FIG. 4. In this embodiment, the controller (in this case aPC) is equipped with both digital in/out (DIO) and A-D converters. TheDIO controls the pseudo-random noise (PRN) generator, which fabricatestwo identical noise-like waveforms, differing only in a user-controlledtime offset. As will be described, this time offset determines thedownrange distance at which the radar is “looking.” Both waveforms arethen upconverted to the chosen frequency band, where one is amplifiedand transmitted, while the delayed version is sent to the correlationreceiver. The transmitted wave scatters from any targets, and isreceived through the antenna array, where it is correlated with theoriginal delayed transmit signal. When the set delay equals thepropagation time in the transmitted signal, a correlation peak occurs,signaling a reflection from a particular distance. The resultantcorrelation is directly proportional to the impulse response of thescattering targets convolved with that of the radar system. This outputvoltage is then recorded using a 12-bit A-D and delivered to thecontroller. One voltage value is recorded for each delay setting.

The parallel processing configuration may provide faster data collectionspeed than the alternate serial switched-antenna configuration, whichmay be desired for a moving vehicle-mounted radar. Each channel (perantenna) may perform the analog correlation simultaneously, onlyrequiring the A-D to operate fast enough to record the output voltage inan approximate 2-millisecond window. In this configuration, the additionof more channels may not increase processing time, only the requirednumber of independent A-Ds.

In this example, the transmission bandwidth chosen was 3.0-3.8 GHz, andresulted as a compromise between foliage penetration requirements,antenna size, and component specifications. This bandwidth conformed toa recent FCC Report and Order that could possibly ban other imagingsystems from operating between 960 MHz and 3.1 GHz. Other constraintsincluding transmission characteristics, size, cost, and complexity mayalso impact the selection of the transmission bandwidth.

FIG. 5 presents a block-diagram for the correlation-type receiverutilized in each channel of the radar of the exemplary embodiment. Inthis example, the major components of the receiver include powerdividers, RF monolithic amplifiers, double-balanced frequency mixers,and op-amps. The local oscillator (LO) of each mixer is driven by thedelayed noise-waveform. The mixers of this embodiment may operateoptimally with a +10 dBm LO signal, but may be useful down to +4 dBm.Since the network of power dividers may add 6 dB of loss (3 dB perdivider), the input signal to the receiver module may range from +10 to+16 dBm in this embodiment. The RF front end of the receiver isamplified (approximately +15 dB) and mixed down to base-band with 8 dBof conversion loss. High-order harmonics are then filtered out, and thefinal-stage BPF produces the voltage-output to be recorded.

The process of mixing and filtering in this example is the analogimplementation of cross correlation between the transmitted and receivedsignals. The correlation value may be directly proportional to theimpulse response of the system, including the cables, antennas andscattering objects. The total recorded time-domain impulse response canthen be represented as,h_(tot)=h_(targets){circle around (x)}h_(radar)where {circle around (x)} is the convolution operator. This is thewaveform that is recorded after correlation by the controller A-Dconverter. To isolate the target response from the radar response,reference measurements may be taken and used in post-processingcalculations.

A schematic of an exemplary receiver module is shown in FIG. 6. In thisexample, the gain of the receiver modules is adjustable viapotentiometers, allowing full utilization of the range of the 12-bit A-Dcard, providing 20*log10(2¹²)=72 dB of dynamic range. An experiment at3.0 GHz showed detection of a target of RCS=42 m², at a 74.86 m range,using a transmit power of 14 dBm and total antenna gain of 16.2 dB. Viathe below radar range equation, the calculated minimum detectable signalfor the radar receiver of this example is −81.5 dBm or 7.13×10⁻⁹ mW.$P_{MIN} = \frac{P_{t} \cdot G_{ANT} \cdot \lambda^{2} \cdot {RCS}}{\left( {4\quad\pi} \right)^{3}R_{MAX}^{4}}$

A target range-tracking experiment using a swept CW transmission (from3.0-3.8 GHz) was performed to verify the receiver capability. The targettracked was the same target used above in the minimum detectable signalcalculations.

A performance factor with the exemplary embodiment was the effect oftemperature-dependent drift with the pre-amp op-amps. The base-lineoutput of the correlators may fluctuate on the order of millivolts,dependent on ambient temperature fluctuations and air movement. Thisfactor can be taken into account through the utilization of highertemperature-rated op-amps, the addition of a cooling fan, and usingchips with only one op-amp per package (to reduce current to each chip).Also, a minimum amount of post-processing through pair-wise subtractionmay be used to account for this performance factor.

An exemplary embodiment of a noise-radar of the present inventiontransmits wide-band, low power noise, and attempts to correlate thereceived signal with an exact copy of the transmitted signal. Randomnoise may have the property of zero time-domain autocorrelation at anyoffset, τ, other than t−τ=t. For this reason, it is an excellent choicefor radar transmission. If a received noise signal correlates with thetransmitted noise that was sent τ=Δt seconds ago, there may be noambiguity in the fact that the total path for the transmitted signalfrom antenna to antenna was also Δt seconds. Using the speed of lightconstant C=2.998×10⁸ m/s allows derivation of the distance the signaltraveled, which may generally be 2 times the distance from the antennasto the scattering target.

Since the use of true white noise (as defined above) may requirephysical delay lines for correlation (or complex and expensive digitalstorage systems) the use of a pseudo-random code may be preferred. If aknown digital word with the same autocorrelation characteristics ofnoise is used for transmission, then a second identical word (with atime offset) may be fabricated for correlation purposes, with no needfor delay elements. Since the word is finite, it may be repeated if theword transmission time is less than the time required to make acorrelation measurement. The act of repeating the PRN waveform mayintroduce an ambiguous downrange phenomenon, since a non-zerocorrelation with the delayed waveform may indicate round-triptransmission time of Δt+nT, where T is the total word transmission-time.The unambiguous range can be calculated by:$R_{u} = \frac{c_{o}N}{2f_{clk}}$where N is the total number of bits in the discrete PRN word, and f isthe bits/second rate of PRN transmission.

An exemplary operational block diagram for a PRN generator is shown inFIG. 7. The exemplary nine-bit shift PRN generators are described inmore detail below. In this configuration, the LOAD command resets onePRN generator, while it loads the other with values from the controller.When the clock is enabled, the top PRN generator starts streaming thedigital noise word from the same place, while the lower one (dependingon its loaded values) will start streaming at some positive time-offset.The effect is two identical PRN waveforms, with a known, settable timedelay between them. The logic diagram of the PRN generators is shown inFIG. 8 as a basic Galois tapped shift-register configuration.

High speed ECL logic was used to implement the noise generator, whileTLL-to-ECL translators were used to interface the design with the DIOcard. In this example, the clock speed used was 760 MHz, and the totalword length using 9-bit shift registers was 2^(N)−1=511 bits. Thisprovided an unambiguous range of 100.8 meters. The smallest possibledifference in delay between the two waveforms was 1/f_(clk)=1.32 ns,which produced a smallest available range-step of 7.75 inches.

From the output of the PRN module and on, these signals are no longerfor these purposes to be considered “digital.” Although they havesquare-wave properties, they can be manipulated in a substantiallyidentical matter to any RF or analog signal. A precision voltageregulator was found to be useful since ECL logic requires very stricttolerances on DC power levels.

Overall, the PRN generator performed well. The output waveforms werewell correlated. In addition, the delay between them was successfullyset.

In this example, two methods were considered to convert the basebandsquare-wave noise signal into a transmittable signal in the chosenbandwidth of 3.0-3.8 GHz. The first method was high-order harmonicfiltering and amplifying, and the second was basic AM modulation.

The largest spectral content of the square-wave noise of this examplewas located between DC and the clock frequency of 760 MHz. However, dueto the fast logic rise and fall times on the order of 300 ps, there wassignificant spectral content around {fraction (1/300)} ps=3.3 GHz. Theseharmonics can be bandpass filtered to 3.0-3.8 GHz, amplified, andtransmitted directly without the need for any other modulation. Thisclock frequency was specifically chosen because it placed the high-orderspectral nulls close to 3.0 and 3.8 GHz, while still utilizing most ofthe 800 MHz bandwidth.

A standard bandpass filter may be completely reflective in the stopband.Since the ECL logic of the PRN generator may be sensitive toElectromagnetic Interference (EMI), the high-power reflections betweenDC and 3.0 GHz may degrade the performance of that module. The avoidthis, a diplexer circuit was designed as shown in FIG. 9. Any spectralcontent below the passband of 3 GHz is shunted to a 50-ohm load toeliminate the high-power lower harmonics reflection.

The module was designed using microstrip techniques. The main componentswere a 5th-order Maximally-flat low-pass filter, a 5th order 0.5 dBEqui-ripple bandpass filter, and a 50-ohm resistor.

While there was noticeable variation between the simulated and measuredresponses, the actual results were better than −13 dB on thelow-frequency S11, while the insertion loss in the passband was betterthan −2 dB. This provided a lower than 1.6 VSWR match up to around 2.5GHz.

The power of the actual high-order harmonics after filtering was nearabout −70 dBm. Since the LO requirement of this example was +7 dBm, andtransmission power was generally around +30 dBm, a system of low-noisehigh gain amplifiers was used to utilize the diplexer.

An AM Modulation Module was designed using standard analog up-conversiontechniques. A Local Oscillator (LO) generated a CW tone centered in theband of transmission, at ƒ₀=3.4 GHz. The two PRN signals were low-passfiltered, and used to modulated the LO waveforms, through twodouble-balanced mixers. As before, the two RF signals were thenamplified and used for transmission and correlation as described.

FIG. 10 is a block diagram of an exemplary embodiment of an upconverter.DC-block capacitors were used to eliminate the characteristic bias so asto suppress the large carrier in mixed output. A conventional lowpassfilter limited the output RF signal to 800 MHz after modulation. Adiplexer was not necessary in this application, since the high-powercomponents were in the passband. With the proper setting of the LO to3.4 GHz, the output transmission spectrum fell between 3.0 and 3.8 GHz.As the schematic of the final PC board layout and the actual circuit inFIG. 11 shows, a potentiometer-tuned VCO was used for the LO generation.

This embodiment of the board performed as expected. The total integratedpower between 3.0 and 3.8 GHz was 10 dBm on both channels.

Antenna Array Design

Regarding the antenna array design, a static antenna array allowed2-dimensional imaging at reduced complexity. This example of the antennaarray provided one separate, isolated antenna element for each isolatedreceiver circuit. In this configuration, many antenna/receiver elementscan span a large physical dimension, providing real-aperture coverageacross potentially the entire front of a Humvee-sized vehicle.

There were four main design characteristics which lead to the finalchoice of the below described Vivaldi Tapered Slot Antenna (TSA) array.The array used wideband response from 3-4 GHz, a low-profile andlow-drag form, appropriate antenna pattern, and minimized cost andcomplexity. For this example, the cost was especially important since upto forty antennas may be required to span the desired physical distance.

The Vivaldi TSA may be fabricated from a single sheet of RF laminate,and may have bandwidth potentials well over the required 1 GHz. In thisembodiment, the average characteristic pattern of the antenna had a mainE-plane beam of 50° and an H-plane beam near 35°. As supported by thepreliminary testing, foliage penetration was particularly effectiveusing horizontal polarization. For effective imaging, each antennaelement may have a wide crossrange beam, but since the targets ofinterest may be on the ground, the vertical beam may be narrow so as toachieve maximum downrange gain. These pattern requirements, and the lowcomplexity and profile of an RF laminate sheet design made the Vivaldiantenna a desirable choice.

For this example, the minimum spacing between the individual antennaelements may be equal to or less than 2 inches to avoid spatialaliasing. This, in turn may limit the widest aperture (see FIG. 12) ofeach antenna element to 2 inches as well.

For proper matching to the 50-ohm receiver system, the horn of theantenna was tapered exponentially to a very small slot width, W. Forthis example, the smallest W may be was about 4 mils, due to fabricationconstraints, which (with the relative dielectric constant of 2.33)created a characteristic impedance of about 80-ohms. The feed for theantenna utilized mutual coupling of a balanced Y-line microstrip/slotinterface. The length with the proper taper and the required W and Hyielded an L of approximately 12 inches. The measured gain of theantennas was approximately 8 dBi. H-plane 3 dB beamwidth was 40°, andthe E-plane beamwidth was 50°.

An antenna isolation measurement was also been performed. Via networkanalyzer tests, first-adjacent antenna isolation was better than 20 dB.Second adjacent and third adjacent antennas had isolations of 30 dB and40 dB respectively.

PC Interfacing and Software Design

One example of the PC interface and software design utilized a PentiumIII Personal Computer (PC) as a controller. A PCI card used for thisapplication was the Measurement Computing model PCI-DAS1001 12-bit A/Dconverter with onboard Digital Input/Output (DIO). National InstrumentsLabview 5.1 was used for interface programming, using MeasurementComputing's Universal Library for Labview.

Two physical interfaces were used for operation. A DIO interface wasused to control the PRN generator, and hence the transmission output,while four A/D channels monitored the output of each channel of thecorrelation receiver. Regarding digital interfacing, a 100-pin to2×50-pin cable was used to translate two 50-pin output ribbon cables tothe one 100-pin card input. A C100FF-2 cable was used for this purpose.

The receiver module had 4 SMA connectorized outputs, which were sampledby the 12-bit A/D portion of the PCI card. The input voltage rangesetting used was about −1.0-1.0 V. The analog inputs used on the cardwere bipolar channels 0-3.

A block diagram of the data acquisition process is shown below in FIG.13. The software used to implement the process was LabView. The mainblock of the program was the hardware-communication (“Set and Record” inFIG. 13) subroutine, which is found in FIG. 14 as a block diagram andFIG. 15 as a timing diagram. The timing diagram shows the actual TTLlevels used to control the transmission of the noise code. Note that thetime delays between B & C and C & D may only need to be a few clockcycles for certainty of data set.

Noise Radar System Testing and Evaluation

The radar system was tested as a system by performing a series of movingradar target tests. The first test was a set of runs where a sweepingmicrowave signal generator was used as the source. In this case, theparallel set of correlation channels for receive were tested. The radarwas operated in the chirp mode, transmitting 20 mW power over the spanfrom 3.0 to 3.8 GHz. Data collected as a function of time/frequency wasprocessed by taking a Fourier transform over the frequency span. Theresult was a range profile. In this test, a person carrying a small (30cm per side) corner reflector moved away at approximately 1 foot persecond. The target could be tracked out to nearly 180 feet with thisembodiment of the system. The receiver sensitivity was acceptable forour tracking application. A single range profile (impulse response) wasobtained by taking an FFT of the detected signal as a function offrequency. Multiple scans were taken, and an earlier (no-target or“background”) scan was subtracted from this scan to remove fixed cluttersources. A large peak at about 200 feet in the data corresponded to theperson carrying the small corner reflector. Remember that there are fourchannels in this exemplary receiver operating in parallel. Data from thefour channels may be used for azimuthal tracking of targets. A displayset of four scans showed that all four channels detected the person withthe small corner reflector at more than about 200 feet.

Next, the pseudorandom noise (pn) generator and the up-converter wereactivated as the signal source in our radar. No modification to thecorrelation receiver was needed. Our first test was to measure theresponse of a 50 foot long coaxial cable. The time shift was scannedover a span from 0 to 200 ns, and the correlation response of the cablewas recorded. A plot of the cable response as measured by the pn radarwas recorded. As expected, the impulse response peak occurred at about75 ns plus the short cable used for the receive antenna. This testshowed that the computer algorithm and interface used to set the timedelay for the shift register time offset was working correctly. The timewidth of the response was approximately 2 ns, corresponding to aneffective radar bandwidth of approximately 500 MHz.

A detail of the response of the pn radar system for a single range gateat 70 feet was graphed. A person carrying a large (1 m) corner reflectorwalked through the set range at approximately 3 feet per second (fps).As a result of this test, the inventors discovered that signalprocessing in the final detection and processing algorithm may be usedto account for a phase interference pattern as a target moves inrelation to the radar.

The inventors also proved that the radar may detect obstacles to amoving vehicle. For example, a small set of “triggers” may be set up asvarious ranges and quickly scanned. The scattering response of a movingtarget through 4 range gates was tracked. In this case, a personcarrying a small corner reflector walked inward toward the radar frombeyond 150 feet. Each time the person passed through a detection zone,there was a strong response. This permitted the software to detect andtrack all of the obstacles ahead of a moving vehicle. In addition, theremay be a large number of antennas so that azimuthal tracking may beincluded. The increase in the number of antennas may not be a timeproblem, because each antenna may have an independent correlationreceiver operating in parallel with all the others.

If the radar system detects a target where more information is needed,then the range gates can be adaptively adjusted by the software to focuson the particular target of interest. In this example, four range gateswere set to cover the zone from 98 to 103 feet from the radar. A targetmoved away and then toward the radar during this observation time. Clearmoving target responses were seen in the data.

Conclusions

The test results demonstrated that the pseudo-noise radar system may beused to alert a moving or stationary vehicle to the presence ofdangerous obstacles. The radar may operate in a frequency band wherepenetration of light (grass and brush) foliage is possible. Thus, theradar may see through fog, rain, snow, darkness, light foliage, and evenbuilding walls. Since the radar may use a very wide band (e.g., greaterthan 800 MHz) spread spectrum signal, it may be very difficult to detectand to jam. Furthermore, noise-like signals may be transmitted overbands that are otherwise licensed to other carriers because it has beenshown that they do not interfere with those carriers.

The radar system and individual components of the present invention havebeen demonstrated herein. In particular, the present invention includes,but is not limited to, a new design for an ultrawide band array antennaand an analog parallel processing receiver system that permits flexiblesoftware control of the system. The radar system may include a displayand/or control system.

The radar system may be installed on a vehicle and used with autonomousobstacle avoidance software. The radar may detect targets including, butnot limited to, people and vehicles moving in relation to the testvehicle (or vice versa) even through light foliage. The radar may alsobe used to detect targets through other types of clutter such asbuilding walls. An exemplary embodiment of the radar system mayrange-triangulate on targets. Thus, rather than a large antenna system,one exemplary system may include only 2 or 4 such antennas and obtainhigh resolution range data from each sensor. Range triangulation mayprovide individual target tracking. Ambiguities may be overcome bytracking each target as the vehicle moves and by cross correlation ofhigh resolution range profiles from the individual sensors. Differenttargets may have different high resolution range profiles.

Any embodiment of the present invention may include any of the optionalor preferred features of the other embodiments of the present invention.The exemplary embodiments herein disclosed are not intended to beexhaustive or to unnecessarily limit the scope of the invention. Theexemplary embodiments were chosen and described in order to explain theprinciples of the present invention so that others skilled in the artmay practice the invention. Having shown and described exemplaryembodiments of the present invention, those skilled in the art willrealize that many variations and modifications may be made to affect thedescribed invention. Many of those variations and modifications willprovide the same result and fall within the spirit of the claimedinvention. It is the intention, therefore, to limit the invention onlyas indicated by the scope of the claims.

1. A radar system comprising: a first memory device storing a first radio frequency waveform; a second memory device storing a second radio frequency waveform; a timing device in communication with said first memory device and said second memory device, said timing device adapted to delay an output of said second radio frequency waveform relative to said first radio frequency waveform; and a first mixer adapted to mix the delayed output of said second radio frequency waveform with a signal that is derived from said first radio frequency waveform interacting with a target.
 2. The radar system of claim 1 wherein said first memory device is a sequential memory device.
 3. The radar system of claim 2 wherein said first memory device is a first-in/first-out circuit (FIFO).
 4. The radar system of claim 1 wherein said second memory device is a sequential memory device.
 5. The radar system of claim 4 wherein said second memory device is a first-in/first-out circuit (FIFO).
 6. The radar system of claim 1 wherein said first radio frequency waveform has a bandwidth between about 100 MHz and 10 GHz.
 7. The radar system of claim 1 wherein said first radio frequency waveform has a bandwidth greater than about 800 MHz.
 8. The radar system of claim 1 wherein said first radio frequency waveform has a bandwidth greater than about 10 GHz.
 9. The radar system of claim 1 further comprising a low pass filter adapted to filter an output of said first mixer.
 10. The radar system of claim 1 further comprising at least one antenna adapted to transmit said first radio frequency waveform and to receive said signal that is derived from said first radio frequency waveform interacting with the target.
 11. The radar system of claim 1 further comprising a computer in communication with said first memory device and said second memory device, said computer adapted to determine said first radio frequency waveform and said second radio frequency waveform.
 12. The radar system of claim 1 further comprising: a first filter in communication with said first memory device, said first filter adapted to filter said first radio frequency waveform; and a second filter in communication with said second memory device, said second filter adapted to filter said second radio frequency waveform.
 13. The radar system of claim 12 wherein: said first memory device is a 1-bit wide first-in/first-out circuit (FIFO); and said second memory device is a 1-bit wide FIFO.
 14. The radar system of claim 1 further comprising: a first digital-to-analog (D/A) converter in communication with said first memory device, said first D/A converter adapted to convert said first radio frequency waveform; and a second D/A converter in communication with said second memory device, said second D/A converter adapted to convert said second radio frequency waveform.
 15. The radar system of claim 14 further comprising: an oscillator adapted to produce a third radio frequency waveform; a second mixer in communication with said first D/A converter and said oscillator, said second mixer adapted to mix an output of said first D/A converter with said third radio frequency waveform; and a third mixer in communication with said first mixer and said oscillator, said third mixer adapted to mix said third radio frequency waveform with a signal that is derived from an output of said second mixer interacting with the target; wherein said first mixer is adapted to mix an output of said second D/A converter with an output of said third mixer.
 16. A radar system comprising: a first 1-bit wide first-in/first-out (FIFO) memory device storing a first radio frequency waveform; a second 1-bit wide FIFO memory device storing a second radio frequency waveform; a timing device in communication with said first memory device and said second memory device, said timing device adapted to delay an output of said second radio frequency waveform relative to said first radio frequency waveform; a first filter in communication with said first memory device, said first filter adapted to filter said first radio frequency waveform; a second filter in communication with said second memory device, said second filter adapted to filter said second radio frequency waveform; and a mixer adapted to mix an output of said second filter with a signal that is derived from an output of said first filter interacting with a target.
 17. The radar system of claim 16 wherein said first radio frequency waveform has a bandwidth greater than about 100 MHz.
 18. The radar system of claim 17 wherein said first radio frequency waveform has a bandwidth between about 100 MHz and 10 GHz.
 19. The radar system of claim 17 wherein said first radio frequency waveform has a bandwidth greater than about 10 GHz.
 20. The radar system of claim 16 further comprising a low pass filter adapted to filter an output of said mixer.
 21. The radar system of claim 16 further comprising at least one antenna adapted to transmit an output of said first filter and to receive said signal that is derived from said output of said first filter interacting with the target.
 22. The radar system of claim 16 further comprising a computer in communication with said first memory device and said second memory device, said computer adapted to determine said first radio frequency waveform and said second radio frequency waveform.
 23. A radar system comprising: a first memory device storing a first radio frequency waveform; a second memory device storing a second radio frequency waveform; a timing device in communication with said first memory device and said second memory device, said timing device adapted to delay an output of said second radio frequency waveform relative to said first radio frequency waveform; a first digital-to-analog (D/A) converter in communication with said first memory device, said first D/A converter adapted to convert said first radio frequency waveform; a second D/A converter in communication with said second memory device, said second D/A converter adapted to convert said second radio frequency waveform; an oscillator adapted to produce a third radio frequency waveform; a first mixer in communication with said first D/A converter and said oscillator, said first mixer adapted to mix an output of said first D/A converter with said third radio frequency waveform; a second mixer in communication with said oscillator, said second mixer adapted to mix said third radio frequency waveform with a signal that is derived from an output of said first mixer interacting with a target; and a third mixer in communication with said second D/A converter and said second mixer, said third mixer adapted to mix an output of said second D/A converter with an output of said second mixer.
 24. The radar system of claim 23 wherein said first radio frequency waveform has a bandwidth greater than about 100 MHz.
 25. The radar system of claim 24 wherein said first radio frequency waveform has a bandwidth between about 100 MHz and 10 GHz.
 26. The radar system of claim 24 wherein said first radio frequency waveform has a bandwidth greater than about 10 GHz.
 27. The radar system of claim 23 wherein: said first memory device is a first-in/first-out circuit (FIFO); and said second memory device is a FIFO.
 28. The radar system of claim 23 further comprising a low pass filter adapted to filter an output of said third mixer.
 29. The radar system of claim 23 further comprising at least one antenna adapted to transmit an output of said first mixer and to receive said signal that is derived from said output of said first mixer interacting with the target.
 30. The radar system of claim 23 further comprising a computer in communication with said first memory device and said second memory device, said computer adapted to determine said first radio frequency waveform and said second radio frequency waveform.
 31. A radar system comprising: a first memory device adapted to store and output at least one waveform; a second memory device adapted to store and output at least one waveform; a timing device in communication with said first memory device and said second memory device, said timing device adapted to delay an output of said second memory device relative to an output of said first memory device; and a mixer adapted to mix said output of said second memory device with a signal that is derived from said output of said first memory device interacting with a target; wherein at least one of said first memory device and said second memory device is adapted to store and output a plurality of different waveforms.
 32. The radar system of claim 31 wherein: said first memory device is adapted to store a plurality of different waveforms; and said first memory device is adapted to output said plurality of different waveforms in a predetermined order.
 33. The radar system of claim 31 wherein: said second memory device is adapted to store a plurality of different waveforms; and said second memory device is adapted to output said plurality of different waveforms in a predetermined order.
 34. A radar system comprising: a first memory device adapted to store and output at least one waveform; a plurality of additional memory devices, each adapted to store and output at least one waveform; a timing device in communication with said first memory device and said additional memory devices, said timing device adapted to delay each output of said additional memory devices relative to an output of said first memory device; and a plurality of mixers, each of said mixers adapted to mix said output of a respective one of said additional memory devices with a signal that is derived from said output of said first memory device interacting with a target.
 35. A radar system comprising: a first memory device adapted to store and output at least one waveform; a plurality of additional memory devices, each adapted to store and output at least one waveform for interacting with a target; a timing device in communication with said first memory device and said additional memory devices, said timing device adapted to delay an output of said first memory device relative to an output of one of said additional memory devices; and a mixer adapted to mix said output of said first memory device with a signal that is derived from said output of one of said memory devices interacting with said target.
 36. The radar system of claim 35 wherein said additional memory devices are adapted to produce output in a predetermined order. 